Acoustic waves can cause variations of the density of a medium in which they travel. The density variations can effect optical gratings. Scattering of an electromagnetic wave by such acoustic gratings in an optical medium is known as "Brillouin scattering". The frequency of the scattered electromagnetic wave in the Brillouin scattering is shifted with respect to that of the original electromagnetic wave due to the Doppler effect by the motion of acoustic waves.
Energy is exchanged in the Brillouin scattering between the optical medium and the electromagnetic wave. Depending on the relative directions of the acoustic wave and the electromagnetic wave, the frequency of the scattered electromagnetic wave may be down-shifted to a lower frequency (i.e., "Stokes shift") or a higher frequency (i.e., "anti-Stokes shift").
Stimulated Brillouin scattering ("SBS") is a nonlinear optical effect which occurs when a coherent electromagnetic wave with an intensity above a certain threshold level is used as a pump in a Brillouin scattering process. See, for example, A. Yariv, Chapter 18, Quantum Electronics, 2nd ed., 1975 (John Wiley & Sons). The nonlinear interaction of the optical medium and the coherent optical pump wave at a frequency .nu..sub.p generates an acoustic wave due to the electrorestrictive effect. This acoustic wave forms a moving acoustic grating in the medium which moves in the same direction of the optical pump wave. The grating scatters the pump wave.
In general, multiple scattered waves are generated by the grating. However, due to the phase-matching restraints, the strongest scattered wave is the back-scattered wave which propagates in the opposite direction of the pump wave. Thus, the frequency .nu..sub.B of the back-scattered optical wave from the acoustic grating is down-shifted relative to the pump wave frequency .nu..sub.p by .nu..sub.D =2nu.sub.a /.lambda..sub.p due to the Doppler effect, where u.sub.a is the velocity of the acoustic wave in the medium, n is the refractive index of the medium, and .lambda..sub.p is the wavelength of the optical pump wave. The remainder of the input pump wave transmits through the medium.
When the input optical power exceeds the SBS threshold, a significant portion of the input power is transferred into the back-scattered optical wave. This results in a saturation behavior in the transmitted wave, i.e., the power of the transmitted wave will no longer increase linearly with the input power. The SBS threshold is known to be linearly proportional to the spectral linewidth of the optical pump wave. Therefore, an optical pump wave with a narrow linewidth can be used to reduce the SBS threshold. For example, in many commercial silica fibers, a SBS threshold of several milliwatts may be achieved by using a pump wave at about 1.3 .mu.m.
When a narrow-band seed signal in the opposite direction of the pump wave with the same frequency of the back scattered wave at .nu..sub.B =(.nu..sub.p -.nu..sub.D) is injected into an optical medium, the interaction between the seed signal and the pump wave can significantly enhance the acoustic grating initially induced by the pump wave. This effect, in turn, increases the back-scattering of the pump wave into the seed signal and thereby effectively amplifies the seed signal. Therefore, the influence of the seed signal converts the spontaneous Brillouin scattering into a stimulated Brillouin scattering, at a pump power much below the SBS threshold. The stimulated back scattering light adds up in phase with the seed signal. This process is known as Brillouin amplification. See, for example, G. P. Agrawal, Nonlinear Fiber Optics, Academic Press, San Diego (1989), Chapter 9.
Brillouin scattering and amplification in optical fibers has been investigated for optical communication applications. See, for example, Olsson and Van Der Ziel, "Characteristics of a semiconductor laser pumped Brillouin amplifier with electronically controlled bandwidth", Journal of Lightwave Technology, Vol. LT-5, No. 1, pp. 147-153 and Tang, "Saturation and spectral characteristics of the stokes emission in the stimulated Brillouin process", Journal of Applied Physics, vol. 37, pp. 2945-2955 (1966). Stimulated Brillouin processes was considered by many as unsuitable for digital fiber optical communication systems at least partially due to its narrow gain bandwidth and high spontaneous emission noise. Many conventional optical communication systems are designed with provisions to suppress the Brillouin scattering in fibers as noise.